Trucks such as dump trucks can be used to haul bulk materials such as dirt, gravel and rock. These materials can be quite heavy. Heavy loads, concentrated on a small wheel base can damage roads and bridges. Government regulations limit the loads that can be carried typically based on the length of the truck (including any trailer) and number of axles. To maximize the amount of load that can be carried by a truck system, truck designers will often add a trailer to increase the length of the truck system, thereby distributing the weight of the load over a greater length. Due to the nature of the weight limit formulas mandated by the government regulations, allowable load weight can be maximized by concentrating load at opposite ends of the truck system, with little or no load in the middle. The result is a truck system comprising a truck with a first load box followed by a trailer with a second load box, the trailer connected to the truck by a long beam called a reach or a tongue.
A transfer dump truck system is a dump truck pulling a separate trailer with a load box that can be moved forward on a frame of the trailer and into the load box of the dump truck. When the transfer dump truck system arrives at its destination, the operator detaches the trailer, drives the dump truck separately to a dump zone where the load is needed and dumps it using the dump cylinder of the dump truck to tilt the dump truck load box. The operator then backs the dump truck over the trailer reach until the back of the dump truck is close to the trailer load box. A load box transfer mechanism pulls the trailer load box into the dump truck load box, which is slightly larger. The load box transfer mechanism is typically a small motor (electrical, pneumatic, or hydraulic). The trailer load box typically moves on small wheels on rails or within guides. The operator then drives to the dump zone and dumps the trailer load box (nested inside the dump truck load box) using the dump truck's dump cylinder. The operator then backs up again over the trailer reach and uses the load box transfer mechanism to move the trailer load box back onto the trailer. This arrangement saves weight and cost, as the load box transfer mechanism is typically much lighter and cheaper than a separate dump cylinder on the trailer to tilt the trailer load box.
The reach of a transfer dump truck system is typically very low to the ground in order to facilitate the dump truck backing over it, as it must to transfer the trailer load box into the dump truck load box. The long, low trailer reach is necessary, but creates a particular problem. When traveling on the highway, the long, low trailer reach is not readily observable to other vehicles on the highway. This is especially a problem when there is another vehicle traveling in the same direction as the transfer dump truck system in a traffic lane to the left of the lane in which the transfer dump truck system is travelling. The driver of the other vehicle, when directly alongside the trailer reach, usually cannot see it, as the view of it is blocked by the passenger side door. To the driver of the other vehicle, the transfer dump truck system may look like two separate vehicles, with an empty space in-between. Worse, positioned alongside the trailer reach, the driver of the other vehicle is likely to have the trailer in a blind spot and not see it at all. This is an unsafe condition as the driver of the other vehicle may attempt to change lanes into the same lane right behind the dump truck, not realizing there is a trailer behind the dump truck. What is needed is a way to warn drivers of vehicles alongside a transfer dump truck system of the presence of a trailer and trailer reach behind the dump truck.